# Geosmin

Geosmin (/dʒiˈɒzmɪn/ jee-OZ-min) is an irregular sesquiterpenoid with a distinct earthy or musty odor, which most people can easily smell. The geosmin odor detection threshold in humans is very low, ranging from 0.006 to 0.01 micrograms per liter in water. Geosmin, along with the irregular monoterpene 2-methylisoborneol, together account for the majority of biologically-caused taste and odor outbreaks in drinking water worldwide and in farmed fish. Geosmin is also responsible for the earthy taste of beetroots and a contributor to the strong scent, known as petrichor, that occurs when rain falls after a spell of dry weather or when soil is disturbed.
In chemical terms, geosmin is a bicyclic alcohol with formula C$_{12}$H$_{22}$O, a derivative of decalin. It is produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step Mg$^{2+}$-dependent reaction. Its name is derived from the Ancient Greek words geō- (γεω-), meaning "earth", and osmḗ (ὀσμή), meaning "smell". The word was coined in 1965 by the American biochemist Nancy N. Gerber (1929–1985) and the French-American biologist Hubert A. Lechevalier (1926–2015).

## Production

Geosmin is produced as a secondary metabolite by various blue-green algae (cyanobacteria), filamentous bacteria in the class Actinomyces, other prokaryotes, and various eukaryotes. The main genera in the cyanobacteria that have been shown to produce geosmin include Anabaena, Phormidium, and Planktothrix, while the main genus in the Actinomyces that produces geosmin is Streptomyces. In beetroots, geosmin is produced endogenously, not by environmental nor endophytic microbes, suggesting beets possess separate or acquired means of geosmin biosynthesis. Communities whose water supplies depend on surface water can periodically experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the local water supply. Under acidic conditions, geosmin decomposes into odorless substances.
In 2006, geosmin was biosynthesized by a bifunctional Streptomyces coelicolor enzyme. A single enzyme, geosmin synthase, converts farnesyl diphosphate to geosmin in a two-step reaction.
Not all blue-green algae produce geosmin. Identification of species that might produce geosmin is traditionally done through microscopic identification of algae as geosmin producers, a technique that is labor-intensive and requires specialized knowledge. Recent advances in molecular biology have enabled identification of a geosmin synthase gene, geoA, which is present in cyanobacterial species that produce geosmin, but is not present in other cyanobacterial species. Amplification of this gene from water samples using real-time PCR may permit predictions of taste and odor events caused by cyanobacteria in fresh water.

## Effects

The human olfactory system is extremely sensitive to geosmin and is able to detect it at concentrations as low as anywhere from 0.4 parts per billion to 5 parts per trillion. It is proposed that this extreme sensitivity to geosmin evolved in humans or their ancestors to aid them in their search for scarce water, but this is not conclusive. Similarly, many insects possess a sensitivity to geosmin such that it may act as an attractant, signaling the presence of microbial prey, or as a repellant, where its presence may coincide with that of toxin-producing microbes. Geosmin is also implicated in the migratory patterns of eels, where its higher concentration in freshwater systems guides the fish from the ocean into estuaries and rivers. Geosmin is also likely involved in raccoon predation of turtle nests, where burrowing disturbs the soil and ejects the chemical and other volatile indicators into the air.
Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as carp, catfish, and tilapia. Geosmin combines with 2-methylisoborneol, which concentrates in the fatty skin and dark muscle tissues. It breaks down in acidic conditions; hence, vinegar and other acidic ingredients are used in fish recipes to reduce the muddy flavor. Taste and odor compounds including geosmin lead to an unpleasant taste of drinking water which is perceived by consumers as an indication of poor water quality. Despite its negative effects on the taste and odor of fish and drinking water, geosmin is nontoxic to humans.
This compound is reported to be an issue for saltwater fish farmed in recirculating aquaculture systems, such as Atlantic salmon, but there are also studies that show that the presence in seawater is significantly lower than that found in freshwater which is why many people consider freshwater fish to taste muddy compared to marine fish. These systems rely on biological filtration using cultured microbial communities to process the nitrogenous waste from the fish (ammonia) into less harmful compounds (nitrite and nitrate) that can be tolerated at higher concentrations. However, geosmin-producing bacteria can also grow in these systems, and often require fish to be transferred to an additional "finishing" or "purge" system where they are not fed for several days prior to harvest to remove off-flavor compounds and empty the intestinal tract. This process is also known as depuration.

## Geosmin remediation in drinking water

Geosmin cannot be removed from water using standard treatment processes. Treating for geosmin requires additional treatment, which may not be available to all municipal water suppliers. Activated carbon filters may be able to help somewhat, but few studies have been completed to address this issue.